Fibroblast growth factors (FGFs) and their receptors (FGFR) are a highly conserved group of proteins with instrumental roles in angiogenesis, vasculogenesis, and wound healing, as well as tissue patterning and limb formation in embryonic development. FGFs and FGFRs affect cell migration, proliferation, and survival, providing wide-ranging impacts on health and disease.
The FGFR family comprises four major types of receptors, FGFR1, FGFR2, FGFR3, and FGFR4. These receptors are transmembrane proteins having an extracellular domain, a transmembrane domain, and an intracytoplasmic domain. Each of the extracellular domains contains either two or three immunoglobulin (Ig) domains. Some FGFRs exist in different isoforms which differ in specific segments of the molecule, such as FGFR-IIIb and FGFR1-IIIc, which differ in the C-terminal region of the third Ig domain. Transmembrane FGFRs are monomeric tyrosine kinase receptors, activated by dimerization, which occurs at the cell surface in a complex of FGFR dimers, FGF ligands, and heparin glycans or proteoglycans. Extracellular FGFR activation by FGF ligand binding to an FGFR initiates a cascade of signaling events inside the cell, beginning with the receptor tyrosine kinase activity.
To date, there are 23 known FGFs, each with the capacity to bind one or more FGFRs (Zhang et al., J. Biol. Chem. 281:15, 694-15,700 (2006)). Several FGFs can bind to and activate each of one or more FGFRs, often with large differences, for order of magnitude differences in their affinities for the different FGFRs. Many FGFs bind their respective FGFRs with very high affinities, some in the picomolar range. Heparin is required for the binding of FGFs to FGFRs under some circumstances (Ornitz et al., Mol. Cell. Biol. 12:240 (1992)). For example, the mitogenic response to FGF-2 (also known as basic FGF (bFGF)) mediated by FGFR1 has been shown to depend on the presence of heparin (Ornitz et al., Mol. Cell. Biol. 12:240 (1992)).
Previously proposed therapeutic approaches using specific antibodies to block FGF function do not address the issue of redundancy within the FGF family in activating multiple FGFRs, since cancers or other proliferative cells may express upregulated levels of more than one FGF or FGFR. Antisense oligonucleotide or related siRNA therapies have potential problems with specificity, serum half-life, and intracellular delivery. Gene transfer therapies, including those using adenovirus, have raised issues of patient safety and a number of clinical gene therapy studies have been halted due to patient death. Small molecule tyrosine kinase inhibitor therapies suffer from issues of target specificity, toxicity, and manifestations of drug resistance. To date, no drug which targets an FGFR signaling pathway has been approved for treating any human disease.